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A few weeks after fertilization, the initial appearance of the human fetal genitalia is basically feminine: a pair of "urogenital folds" with a small protuberance in the middle, and the urethra behind the protuberance.

If the fetus has testes, and if the testes produce testosterone, and if the cells of the genitals respond to the testosterone, the outer urogenital folds swell and fuse in the midline to produce the scrotum; the protuberance grows larger and straighter to form the penis; the inner urogenital swellings swell, wrap around the penis, and fuse in the midline to form the penile urethra.

Because of their common origin in fetal anatomy, a number of male and female anatomical features are said to be homologous; for example, the clitoris and penis are homologous with one another, as are Skene's glands in females and the prostate gland in males.

Most of the time, the result of fetal genital development follows the stereotypical male or stereotypical female development path. However, in a small but significant minority of cases, the path of development follows an intermediate or other pathway, leading to what are called "ambiguous genitalia", one condition of those known as intersexuality.